FIG. 6 is a structural view of a general type of a conventional electric discharge machine.
In FIG. 6, numeral 1 designates an electrode, numeral 2 designates a workpiece, numeral 3 designates a machining fluid, numeral 4 designates a machining tank, numeral 5 designates an electrode rotating device rotating the electrode 1 around-a Z axis, numeral 6 designates a Y-axis table, numeral 7 designates a Y-axis driving device driving the Y-axis table, numeral 8 designates a X-axis table, numeral 9 designates a X-axis driving device driving the X-axis table, numeral 10 designates a Z-axis driving device driving the electrode rotating device 5 attached with the electrode 1 in Z-axis direction, numeral 11 designates an electric source supplying pulses between the electrode 1 and the workpiece 2, numeral 12 designates a machining state detecting device detecting a machining state in machining, numeral 13 designates a machining fluid supplying device supplying the machining fluid to the gap of machining and numeral 14 designates a NC control device. FIG. 7 is a block diagram for explaining the operation of the electric discharge machine shown in FIG. 6. In FIG. 7, parts 11, 12, 13 and 14 are the same as those in FIG. 6. Numeral 15 designates a machining condition setter setting various machining conditions to the electric source 11, the machining fluid supplying device 13, a machining path designator 16, a jump motion controller 17 and a comparator 18. The numeral 16 designates the machining path designator generating a path for machining the workpiece in a desired shape, an electrode planetary pattern and the like, the numeral 17 designates the jump motion controller for having the electrode 1 rise and fall during the machining operation, the numeral 18 is the comparator, a numeral 19 designates a machining controller and numeral 20 designates a machining/jump motion switcher. The operation of these parts 15 through 20 is generally realized by a program in the NC control device 14. Numeral 21 designates an electrode driving device which is constituted by the electrode rotating device 5, the respective axis tables and the respective axis driving devices 6 through 10. Numeral 22 designates a discharge machining process indicating a discharge machining phenomenon caused between the electrode 1 and the workpiece 2 opposedly arranged in the machining fluid 3.
Next, an explanation will be given of the operation.
In a normal electric discharge machine a gap distance control system is constituted for adjusting a gap between the electrode 1 and the workpiece 2 for machining the workpiece in a desired shape while maintaining a stable machining state. The control system compares a reference instruction value set by the machining condition setter 15 with a detected value indicating the electric discharge machining process 22 that is detected by the machining state detecting device 12, by the comparator 18, calculates a deviation and issues an electrode movement instruction based on an instruction from the machining path designator 16 such that the deviation is nullified by the machining controller to thereby control the gap between the electrode 1 and the workpiece 2. Further, the machining is finished at a time point where the electrode movement instruction value becomes a final instruction value of the desired shape. In this case machining is selected in the machining/jump motion switcher 20.
The NC control device 24 has a function of the jump motion control as well as a function of the gap distance control. In the jump motion the machining/jump motion switcher 20 forcibly switches the gap distance control to the jump motion whereby the electrode 1 is risen and fallen. This jump motion is important in view of stabilizing the machining state by evacuating debris from the gap of machining by its pumping operation.
However, in such an electric discharge machine a large positive pressure or negative pressure (hereinafter reactive force by working or working reaction) is operated on the electrode in rising or falling of the electrode in the jump motion, in case where the electrode is especially large or the gap of machining is very narrow as in a finishing operation or the depth of machining is large whereby the main body of the electric discharge machine is deformed and the machining accuracy is deteriorated. According to a research by Mohri et al at Toyoda Institute of Technology "Study on The Characteristics of Electrical Discharge Machining (EDM) in Real Operation", Journal of the Japan Society of Electrical-Machining Engineers, vol. 20, No. 39, p.19-29, 1987, the above-mentioned force operating on the electrode is caused by the viscosity of a machining fluid and a force operating on the electrode when the electrode is falling, especially causes the deterioration of the machining accuracy.
FIGS. 8A, 8B and 8C illustrate a main shaft displacement, a working reaction and a column displacement in a jump motion actually measured by Mohri et al. As is apparent from a portion A in the figures the working reaction is maximized when the electrode is falling. Incidentally, in these figures the main shaft designates the Z axis and the column indicates the main body of the machine supporting the Z axis, respectively and the working reaction is measured by a force sensor integrated in an electrode attaching jig.
To solve the above-mentioned problem Mohri et al proposed that the rigidity of the machine is to be enhanced by a planer-type structure and the working reaction is to be alleviated by reducing an electrode falling speed immediately before the falling of the electrode is finished to thereby decrease the deformation of the column.
A portion B in FIG. 8 shows that the working reaction is smaller than that in the portion A and hence the amount of displacement of the column is reduced. This is due to the decelerated falling speed of the main shaft in the portion B that is a result supporting the proposal of Mohri et al.
Japanese Examined Patent Publication No. 31806/1992 discloses a method of controlling an electrode speed in the jump motion based on the same conception. As shown in FIG. 9 in this method the speed is changed in rising and falling of the electrode in accordance with a distance between the electrode and the workpiece. In FIG. 9, in rising of the electrode the electrode rising speed is accelerated from v2 to v1 at a distance L1 between the electrode and the workpiece and in falling of the electrode the electrode falling speed is decelerated from v1 to v2 at the distance L1 between the electrode and the workpiece by which the positive pressure and the negative pressure operating on the electrode are alleviated.
Incidentally, in the paper of Mohri et al the area of the electrode is described only up to approximately 20 cm.sup.2. FIGS. 10A and 10B indicate the main shaft displacement and the column displacement in a finishing operation while performing a jump operation in which an electrode having the area of the electrode of approximately 1000 cm.sup.2 is used. In the jump operation the electrode falling speed is controlled to decelerate immediately before the electrode falling is finished. Therefore, although a column displacement is caused in rising of the electrode, almost no column displacement is caused in falling the electrode. However, a noteworthy point in FIG. 10 in comparison with FIG. 8 is that a large column displacement is caused at portions C during time periods in which electric discharge is generated between the electrode and a workpiece (hereinafter, in discharging). There has been no description with regard to the phenomenon in which the column displacement is caused in discharging.
The column displacement shown here is caused not only by a force due to the viscosity of a machining fluid which has conventionally been recognized but by a pressure of a great number of bubbles generated at that occasion by vaporizing the machining fluid confined in the gap of machining by a continuous electric discharge. Therefore, the column deformation is not limited to that in the jump operation. What influences on the actual machining accuracy is predicted to be rather the column displacement in discharging mentioned here than the above-mentioned column displacement in rising and falling of the electrode.
As stated above the conventional electric discharge machine cannot deal with the force received by the electrode by accumulating the bubbles of the machining fluid generated in discharging at the gap between the electrode and the workpiece. Accordingly, the main body of the electric discharge machine is deformed by the pressure of the bubbles causing a work shape error due to the deformation of X, Y and Z axes to be orthogonal to each other and a working dimension error due to the change of the reference position per se and accordingly a sufficient machining accuracy cannot be realized. Further, the machining is finished at a time point when the electrode movement instruction value becomes the final instruction value of the desired shape without considering the deformation of the main body of the machine and accordingly, the work shape error is caused. Moreover, the force acting on the electrode operates as a disturbance to the gap distance control system and a stable machining state cannot be maintained when the pressure is rapidly changed in case where the bubbles are detached from the gap of machining or the bubbles are liquefied again, giving rise to the lowering of the machining speed.